1. Field of the Invention
The present invention relates to a multilayer ceramic substrate, a method for manufacturing the same, and an electronic device including such multilayer ceramic substrate. Particularly, the present invention relates to an improvement for preventing warping which may occur in a multilayer ceramic substrate manufactured by a so-called non-shrinkage process.
2. Description of the Related Art
A multilayer ceramic substrate includes a plurality of laminated ceramic layers. Such multilayer ceramic substrate is provided with various types of wiring conductors. The wiring conductors include internal conductive films extending along predetermined interfaces between the ceramic layers inside the multilayer ceramic substrate, via-hole conductors extending to penetrate a predetermined ceramic layer, and external conductive films extending on the outer surface of the multilayer ceramic substrate.
A multilayer ceramic substrate is used for mounting thereon semiconductor chip components or other chip components and interconnecting these electronic components. The above-described wiring conductors serve as an electric path for this interconnection.
Passive components such as capacitor elements and inductor elements may be incorporated in a multilayer ceramic substrate. In this case, these passive components are provided by part of the internal conductive films and the via-hole conductors as the above-described wiring conductors.
A multilayer ceramic substrate is used as, for example, an LCR complex high-frequency component in the field of mobile communication terminal equipment. It also used as a complex component of an active element such as a semiconductor IC chip and a passive element such as a capacitor, an inductor or a resistor in the field of computers. It also used simply as a semiconductor IC package.
A multilayer ceramic substrate is widely used to constitute various electronic components such as a PA module substrate, an RF diode switch, a filter, a chip antenna, various packaged components and a composite device.
In order to achieve a multilayer ceramic substrate having further multifunctions, high-density and high-performance, it is effective to densely arrange the wiring conductors as described above.
In order to obtain a multilayer ceramic substrate, it is essential to carry out a firing step. However, shrinkage in the firing step due to sintering of the ceramic may occur. The shrinkage is not likely to occur evenly in the whole multilayer ceramic substrate, and thus undesired deformation and warping may occur in wiring conductors. Any deformation and warping in such wiring conductors inhibits high-density of the above-described wiring conductors.
The application of a so-called non-shrinkage process is suggested, wherein shrinkage in the principal surface direction of a multilayer ceramic substrate does not substantially occur during the firing step to manufacture a multilayer ceramic substrate.
In a method for manufacturing a multilayer ceramic substrate using a non-shrinkage process, a low-temperature-sinterable ceramic material which can sinter at about 1000xc2x0 C. or less is prepared. Also, an inorganic material functioning to constrain shrinkage and which does not sinter at the sintering temperature of the above-described low-temperature-sinterable ceramic material is prepared. In order to prepare a green laminate which becomes a multilayer ceramic substrate by sintering, green constraining layers including the inorganic material are placed to contact a principal surface of a predetermined layer of a plurality of laminated green base layers containing the low-temperature-sinterable ceramic material, and wiring conductors are provided in association with the green base layers.
The green laminate prepared in the way described above is then fired. In the firing step, the inorganic material contained in the green constraining layers does not substantially sinter, and thus shrinkage does not substantially occur in the green constraining layers. Accordingly, the green constraining layers constrain the green base layers and as a result, the green base layers substantially shrink only in the thickness direction but shrinkage in the principal surface direction is constrained. Consequently, uneven deformation is unlikely to occur in the multilayer ceramic substrate produced by sintering the green laminate, and undesired deformation and warping is unlikely to occur in the wiring conductors. Therefore, high-density of the wiring conductors may be achieved.
However, when the bonding force between the green base layer and the green constraining layer in the above-described firing step is insufficient, the constraining force of the green constraining layers does not have sufficient effect on the green base layers, and thus sufficient shrinkage constraining effect may not be obtained. In this case, undesired deformation such as warping may occur in the produced multilayer ceramic substrate.
Accordingly, it is an object of the present invention to provide a method for manufacturing a multilayer ceramic substrate which can solve the above-described problems, a multilayer ceramic substrate produced by the method for manufacturing, and an electronic device including such multilayer ceramic substrate.
The present invention is directed to a method for manufacturing a multilayer ceramic substrate, comprising: a laminate preparing step of preparing a green laminate having a plurality of laminated green base layers containing low-temperature-sinterable ceramic material, a green constraining layer placed to contact a principal surface of a predetermined layer of the green base layers and containing inorganic material which does not sinter at the sintering temperature of the low-temperature-sinterable ceramic material, and wiring conductors provided in association with the green base layers; and a firing step of firing the green laminate at a temperature at which the low-temperature-sinterable ceramic material sinters. The present invention includes following features in order to solve the above-described technical problems.
The low-temperature-sinterable ceramic material and the inorganic material are selected so as to chemically react with each other in the firing step to form a reaction layer along an interface between the green base layer and the green constraining layer.
The above-mentioned reaction layer refers to a part wherein a component of the low-temperature-sinterable ceramic material contained in the green base layers and a component of the inorganic material contained in the green constraining layers are mixed at an atomic level. This should be distinguished from a state wherein part of the low-temperature-sinterable ceramic material contained in the green base layers is simply penetrated into the green constraining layers according to capillarity action.
In the above-described mixture at an atomic level, a new crystal phase may or may not be formed from a component contained in the low-temperature-sinterable ceramic material and a component contained in the inorganic material.
When a crystal phase is not formed, a component contained in one of the low-temperature-sinterable ceramic material and the inorganic material may diffuse, dissolve or form a solid solution in a glassy or amorphous phase, or a crystal phase included in the other of the low-temperature-sinterable ceramic material and the inorganic material.
Accordingly, the combination of low-temperature-sinterable ceramic material contained in the green base layers and inorganic material contained in the green constraining layers is important for a formation of a reaction layer.
For example, when the low-temperature-sinterable ceramic material contains borosilicate glass and the inorganic material contains at least one compound selected from the group consisting of spinel, mullite, magnesia, zirconia, zinc oxide, nickel oxide, lanthanum oxide, cobalt oxide, chromium oxide, titanium oxide, iron oxide, calcium oxide, silicon oxide, silicon carbide, boron carbide, tungsten carbide, silicon nitride and boron nitride, or when the low-temperature-sinterable ceramic material contains alumina and the inorganic material contains magnesia or cobalt oxide, a reaction layer wherein a new crystal phase is formed as described above may be formed.
On the other hand, when the low-temperature-sinterable ceramic material contains borosilicate glass and the inorganic material contains alumina powder having the particle size of about 100 nm or less, the alumina may dissolve in a glassy phase in the borosilicate glass contained in the low-temperature-sinterable ceramic material so as to form a reaction layer.
In order to ensure formation of the reaction layer applying the present invention, it is preferable that a programming rate (temperature increase rate) is about 15xc2x0 C./min. or less and a hold time at the maxinum temperature is more than about 10 minutes or more in the firing step.
When the green constraining layer included in the green laminate prepared by the laminate preparing step is placed at each end in the laminating direction of the laminate, it is preferable that at least part of the reaction layer and the green constraining layers are removed after the firing step.
The method for manufacturing a multilayer ceramic substrate according to the present invention may further comprise a step of mounting electronic components on the outer surface of the laminate after the firing step.
The present invention is also directed to a multilayer ceramic substrate produced by the method for manufacturing as described above.
The present invention is also directed to an electronic device including the above-described multilayer ceramic substrate and a motherboard for mounting the multilayer ceramic substrate.
According to the present invention, following advantages will be provided.
In manufacture of a multilayer ceramic substrate using a so-called non-shrinkage process, low-temperature-sinterable ceramic material contained in green base layers and inorganic material contained in green constraining layers chemically react each other during the firing step. In the firing step, a reaction layer is generated by a chemical reaction of the low-temperature-sinterable ceramic material and the inorganic material and is formed along an interface between the green base layer and the green constraining layer. The reaction layer may act to enhance a bonding force between the green base layer and the green constraining layer.
Accordingly, the constraining force of the green constraining layers acts sufficiently on the green base layers in the firing step and the green constraining layers have an excellent shrinkage constraining effect on the green base layers. As a result, shrinkage in the principal surface direction of the green base layers is sufficiently constrained, and thus undesired deformation such as warping may be unlikely to occur in a produced multilayer ceramic substrate.
Therefore, a multilayer ceramic substrate which is small-sized and having high-density in wiring may be produced with enhanced reliability.
In the present invention, when a programming rate in a firing step is about 15xc2x0 C./min. or less, the possibility of forming the above-described reaction layer becomes higher.
Also, when a hold time at the maximum temperature in a firing step is about 10 minutes or more, the possibility of forming the reaction layer becomes higher.
When an electronic device is constituted by mounting a multilayer ceramic substrate produced by the manufacturing method according to the present invention on a motherboard, miniaturization and high-density in wiring of the multilayer ceramic substrate are achieved, and deformation such as warping is not likely to occur, as described above. Therefore, the miniaturization and multifunction of such electronic device may advantageously be achieved. In addition, reliability to a connection between the multilayer ceramic substrate and the motherboard in the electronic device may be increased.
Further objects, features, and advantages of the present invention will be apparent from the following description of the preferred embodiments with reference to the attached drawings.